﻿using System;
using System.Text;

public static class StringBuilderSubstring
{
    public static StringBuilder Substring(this StringBuilder input, int index, int lenght)
    {
        StringBuilder substring=new StringBuilder();
        int end = lenght + index;
        for (int i = index; i < end; i++)
        {
            substring.Append(input[i]);
        }
        return substring;
    }
}